Jet noise-reduction system

ABSTRACT

System includes producing temporary shield in form of elongate layer of cold gas extending rearwardly of engine exhaust nozzle in adjacency with lower portion of exhaust gas steam. Shield may be flat but is preferably shaped concave upward to surround lower half of exhaust stream. Shield produced by forcefully projecting a stream of pressurized evaporable fluid which evaporates to form a layer of substantial mass and thickness which reflects at least some sound waves upwardly. Cools and mixes with adjacent exhaust stream to increase mass and cool the stream to reduce sound energy. Mass of shield also absorbs and refracts some of the sound energy. Apparatus is manifold having rear facing outlet means and supplied by pressurized store of liquid or cryogenic fluid carried by nacelle. Pressure pump may be used for liquids such as water.

United States Patent Inventor Howard R. Macdonald San Diego, Calif. Appl. No. 826,867 Filed May 22, 1969 Patented Nov. 9, 1971 Assignee Rohr Corporation JET NOISE-REDUCTION SYSTEM 6 Claims, 2 Drawing Figs.

U.S. Cl 181/33 11C, 181/33 B, 181/51, 239/265.l7 Int. Cl 864d 33/06, F0ln 1114 Field olSearch 181/3143, 51, 33.05, 33.02, 33.221, 33.222, 33.223; 239/1273, 265.11, 265.13, 265.17, 265.19, 265.23, 265.33

References Cited UNITED STATES PATENTS 0,905 7/1961 Lilley 181/3322] 7,710 4/1962 Maytner.... 181/3322] 4,282 3/1965 Harrison 181/3322! 3,294,323 12/1966 Ernst 239/265.23 FOREIGN PATENTS 1,157,063 12/1957 France Nil/33.221

1,436,412 3/1966 France Nil/33.221

Primary Examiner- Robert S. Ward, .lr. Attorney-George E. Pearson ABSTRACT: System includes producing temporary shield in form of elongate layer of cold gas extending rearwardly of engine exhaust nozzle in adjacency with lower portion of exhaust gas steam. Shield may be flat but is preferably shaped concave upward to surround lower half of exhaust stream. Shield produced by forcefully projecting a stream of pressurized evaporable fluid which evaporates to form a layer of substantial mass and thickness which reflects at least some sound waves upwardly. Cools and mixes with adjacent exhaust stream to increase mass and cool the stream to reduce sound energy. Mass of shield also absorbs and refracts some of the sound energy. Apparatus is manifold having rear facing outlet means and supplied by pressurized store of liquid or cryogenic fluid carried by nacelle. Pressure pump may be used for liquids such as water.

PATENTEUuuv 9 I97! INVENTOR.

BY HOWARD R. MACDONALD vzmuw ATTORNEY 1 JET NOISE-REDUCTION SYSTEM BACKGROUND OF THE INVENTION This invention lies in the field of gas turbine engines, which produce reaction thrust by ejecting a high-velocity stream of gas from the exhaust nozzle of the gas turbine. This stream creates a very high level of sound energy, in a wide range of frequencies and a portion of this sound energy or "noise" reaches the ground from low flying jet airplanes at a noise level which is not acceptable to the public. Many schemes have been propoud for reducing this noise, including modifications of the engine system itself and various types of attachments cooperating with the exhaust nozzle to modify the character of the exhaust stream or to absorb or deflect some of the sound waves.

The invention is directed more particularly to the class of sound or noise reducers which seek to absorb or reflect some of the sound waves, or both. Various devices of this class, generally referred to as shields, have beenproposed andtried with some degree of success. However, they have limitations and disadvantages which reduce their theoretical .utility.

.Usually such device consists of a flat or curved shield, usually of metal, which is structurally supported by the enginenacelle,

pylon, or wing. Since it is exposed to the veryhigh-velocity.

ambient air slipstream, it must be very strong and rugged and must be securely mounted. In addition, since it produces a great deal of drag it must be retracted during high-altitude flight. All of this results in a serious weight penalty since the structure must be carried throughout the flight although it is used only for a few minutes.

If the shield is also intended to absorb sound, it must be provided with sound-absorbing material and mounting means which further add to the total weight. The shearing action of the high-velocity exhaust stream with respect tovthe. ambient air, which produces the objectionable noise, extendsfar-tothe rear of the exhaust nozzle. This requires that the shield be extended as far rearwardly as is reasonably possible, resulting in maximum weight and stowage problems. Even the longest shield can deal only with a small initial portion of the exhaust stream.

SUMMARY OF THE INVENTION The present invention overcomes the difficulties mentioned above and provides a noise-reduction system which is quite effective in action while adding very little weight to the airplane except at take off and almost completely eliminating structural and operational complications. Generally stated, a shield which is temporary in nature is produced by forcefully projecting a stream of evaporable fluid rearwardly with respect to the direction of travel to form an elongate blanket or layer of evaporated gas in adjacency with the lower portion of the exhaust gas stream. While the layer dissipates gradually it maintains its integrity long enough to efiectively create a shield extending many airplane lengths rearwardly of the source.

The layer may be relatively flat but preferably is formed to havea cross section which is concave upwardly to surround substantially the lower half of the cylinder or cone of the exhaust stream. It may be directed parallel to the exhaust stream or angled slightly upwardly and rearwardly to intercept the exhaust stream. In preferred form, the apparatus for producing the shield takes the form of a manifold having a curved por tion generally conforming to the contour of the lower portion of the cell, the manifold being provided with single or multiple outlet means to project pressurized fluid rearward for evaporation and expansion into a substantially homogeneous layer. The manifold is supplied from a storage tank carried in the nacelle or wing. To eject the fluid used, the tank may be pressurized or a mechanical pump may be added, or both.

The cold gas in the shield mixes with the adjacent ambient air and the adjacent portion of the exhaust stream, cooling any moisture contained in them and increasing the mass and density of the shield. The latter acts in much the same way as a solid body in reflecting some of the sound waves emitted by the exhaust stream upwardly. In addition, it absorbs and refracts other portions of the sound waves. The cooling of even a portion of the exhaust stream reduces the amount of sound emitted, as isknown in the art. All of these functions together greatly reduce the sound energy or decibel level at the ground. The heaviest component of the system is the fluid, which is expended during takeoff. Consequently the remaining load which must be carried with the airplane consists of a very light manifold, an empty tank, and possibly a pump. Thus it will be seen that the function is accomplished with a minimum of weight, drag, and structural and operational complication.

BRIEF DESCRIPTION OF THE DRAWINGS Various other advantages and features of novelty will become apparent as the description proceeds in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic side elevational view of a jet engine mounted on a wing with the system of the invention installed and operating; and

FIG. 2 is a sectional view taken on line 2-2 in FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT The total system is generally illustrated in schematic fashion in FIG. 1, in which a jet engine nacelle 10 containing a jet engine, not shown, is carried by a pylon or faired support 12, which in turn is mounted to wing 14. The nacelle is tapered slightly and terminates at its aft end 16 in a plane substantially coterminous with the exhaust nozzle 18. The slightly expanding cylinder or cone of the exhaust gas stream 20 is indicated by numeral 22.

In order to produce the desired gaseous shield, a manifold 24 is provided having rearwardly directed outlet means preferably in the form of nozzles 26 sufi'iciently closely spaced that the individual jets issuing therefrom will quickly expand to form a homogeneous layer or blanket 28 of gaseous material. A storage tank 30 containing evaporable fluid is connected to manifold 24 by a suitable conduit 32 provided with a remotely controlled valve 34. If the fluid is a cryogenic gas such as nitrogen or carbon dioxide, it may be stored under pressure in the tank to reduce the equipment necessary. Alternatively, the fluid (cryogenic, water or some other suitable liquid) in the tank may be pumped out, for which purpose a pump 36 is interposed in the conduit line and powered by a suitable motor 38. When water is used it may be treated with an antifreeze, or an inhibitor to facilitate its storage and its action with the exhaust stream. The manifold could be a flat pipe extending laterally at the lower edge of the nacelle but preferably it is curved arcuately into about a half circle and streamlined into the contour of the nacelle as shown. Also the outlet means could be a continuous slot but the separate nozzles 26. give better control of the pressurized fluid.

As the pressurized fluid leaves nozzles 26 it evaporates and expands to form a substantially homogeneous layer of gaseous material which, in the preferred form, is shaped to be concave upward as best seen in FIG. 2. The shield 28 thus formed has an interface 40 with the ambientair and an interface 42 with the exhaust gas stream. At interface 40 the relative motion causes mixing of shield 28 with the adjacent air and cooling of any moisture in the air, thus increasing the mass and density of the shield. The same result occurs at interface 42 with the exhaust gas stream. An additional benefit is that cooling of the adjacent portion of the exhaust stream by the shield reduces the amount of sound energy produced.

Because of the difference in density between the exhaust stream and the shield, the latter acts somewhat like a solid body and reflects upwardly at least some of the sound waves which would otherwise be projected to the ground. The shield also operates to absorb some additional sound waves and refract others in known manner.

It will thus be seen that the system of this invention reduces the total sound energy projected toward the ground from an airborne jet engine in four ways; that is, by cooling the exhaust stream, and by reflecting, absorbing, and refracting sound waves. The major part of the weight of the installation is expendable material which must be carried only a short distance and which represents only a minute portion of the weight of the airplane or its fuel. Moreover the system is extremely simple and trouble free.

In some circumstances it may be desirable to produce a more positive interception of the exhaust stream by the shield. For this purpose the manifold may be mounted at an appropriate angle, or the nozzles may be so mounted that they are directed a few degrees upwardly and rearwardly.

What is claimed as new and useful and is desired to be secured by US. Letters Patent is:

l. A method of reducing the sound energy transmitted to the ground from the exhaust gas stream emitted by an airborne jet engine, comprising: producing a temporary shield by forcefully projecting rearwardly from an external zone of the nozzle a stream of pressurized evaporable liquid directed along the underside of the exhaust gas stream from the engine in the fonn of an elongate layer of cold gas radially externally of the exhaust gas stream, extending said shield rearwardly of the exhaust nozzle of the engine along the underside of the exhaust stream; and utilizing the mass of said shield to reflect upwardly at least some of the downwardly directed sound waves emitted by the exhaust gas stream.

2. A method of reducing the sound energy transmitted to the ground from the exhaust gas stream emitted by an airborne jet engine, comprising: producing a temporary shield by forcefully'projecting rearwardly from an external zone of the nozzle a stream of pressurized cryogenic fluid directed along the underside of the exhaust gas stream from the engine in the fonn of an elongate layer of cold gas radially externally of the exhaust gas stream, extending said shield rearwardly of the exhaust nozzle of the engine along the underside of the exhaust stream; and utilizing the mass of said shield to reflect upwardly at least some of the downwardly directed sound waves emitted by the exhaust gas stream.

3. Apparatus for producing a temporary, gaseous, directional, sound-reducing shield extending rearwardly of the exhaust nozzle of a jet engine and adjacent the lower portion of the exhaust gas stream emitted thereby, comprising: a fluid manifold adapted to be mounted adjacent said engine and provided with outlet means located and directed to emit fluid in the form of a generally horizontally extending gaseous sound shield along the underside of the engine exhaust gas stream; and means to supply pressurized fluid to said manifold.

4. Apparatus as claimed in claim 3 said outlet means comprising a row of closely spaced nozzles located and directed to emit individual streams of fluid in sufl'icientiy closely spaced relation to cause them to merge into a substantially homogeneous, gaseous layer.

5. Apparatus as claimed in claim 3 said outlet means being formed to emit a layer of fluid having an upwardly concave cross section conforming substantially to the underside of the engine exhaust gas stream.

6. Apparatus for producing a temporary, gaseous, soundreducing shield rearwardly of the exhaust nozzle of a jet engine and in adjacency to the lower portion of the exhaust gas stream emitted thereby, comprising a fluid manifold adapted to be mounted adjacent said engine and located forward of the outlet of the exhaust nozzle, said manifold being provided with rearwardly directed outlet means to emit a stream of evaporative fluid in the form of a generally horizontally extending shield overlapping said exhaust nozzle, and means to supply pressurized fluid to said manifold.

8 t I 18 l 

1. A method of reducing the sound energy transmitted to the ground from the exhaust gas stream emitted by an airborne jet engine, comprising: producing a temporary shield by forcefully projecting rearwardly from an external zone of the nozzle a stream of pressurized evaporable liquid directed along the underside of the exhaust gas stream from the engine in the form of an elongate layer of cold gas radially externally of the exhaust gas stream, extending said shield rearwardly of the exhaust nozzle of the engine along the underside of the exhaust stream; and utilizing the mass of said shield to reflect upwardly at least some of the downwardly directed sound waves emitted by the exhaust gas stream.
 2. A method of reducing the sound energy transmitted to the ground from the exhaust gas stream emitted by an airborne jet engine, comprising: producing a temporary shield by forcefully projecting rearwardly from an external zone of the nozzle a stream of pressurized cryogenic fluid directed along the underside of the exhaust gas stream from the engine in the form of an elongate layer of cold gas radially externally of the exhaust gas stream, extending said shield rearwardly of the exhaust nozzle of the engine along the underside of the exhaust stream; and utilizing the mass of said shield to reflect upwardly at least some of the downwardly directed sound waves emitted by the exhaust gas stream.
 3. Apparatus for producing a temporary, gaseous, directional, sound-reducing shield extending rearwardly of the exhaust nozzle of a jet engine and adjacent the lower portion Of the exhaust gas stream emitted thereby, comprising: a fluid manifold adapted to be mounted adjacent said engine and provided with outlet means located and directed to emit fluid in the form of a generally horizontally extending gaseous sound shield along the underside of the engine exhaust gas stream; and means to supply pressurized fluid to said manifold.
 4. Apparatus as claimed in claim 3 said outlet means comprising a row of closely spaced nozzles located and directed to emit individual streams of fluid in sufficiently closely spaced relation to cause them to merge into a substantially homogeneous, gaseous layer.
 5. Apparatus as claimed in claim 3 said outlet means being formed to emit a layer of fluid having an upwardly concave cross section conforming substantially to the underside of the engine exhaust gas stream.
 6. Apparatus for producing a temporary, gaseous, sound-reducing shield rearwardly of the exhaust nozzle of a jet engine and in adjacency to the lower portion of the exhaust gas stream emitted thereby, comprising a fluid manifold adapted to be mounted adjacent said engine and located forward of the outlet of the exhaust nozzle, said manifold being provided with rearwardly directed outlet means to emit a stream of evaporative fluid in the form of a generally horizontally extending shield overlapping said exhaust nozzle, and means to supply pressurized fluid to said manifold. 